This invention relates to a method of printing fine patterns by which fine patterns are formed by printing processes. More specifically, the invention relates to a printing method suitable for forming, with high precision and moreover on a quantity-production scale, fine resist patterns of, for example, patterns of fine electrical circuits and elements to be supplied for electronic components.
In recent years, great progress has been made in the formation of printed electrical wiring and circuit patterns on glass and ceramic plates. Advancement has also been made in the formation of resist patterns for etching of metal plates. In these methods of forming patterns, printing techniques such as screen printing and offset printing are widely adopted. These printing processes having been suitable for forming patterns of relatively wide (200 .mu.m or more) printing elements (lines). However they have not been suited to the formation of fine patterns of printing areas of less width. Moreover, these prior techniques have been accompanied by problems such as deformation of printed patterns due to causes such as the effect of the fluidity of the ink and the pressure of the printing plate and remnant ink on the printing plate and not being transferred onto the object being printed. As a consequence, the reproducibility of the printed pattern has been poor.
For example, by the screen printing process, printing is carried out by forming on a mesh-form screen an ink-shielding mask so that the non-masked parts of this mask form a desired pattern and causing ink to pass through the non-masked parts and adhere to the object being printed (hereinafter referred to as print object). By this printing method, thick ink impression (thickness of a number of .mu.m to 20 .mu.m) can be readily achieved. For this reason, printing of an excellent resist pattern having erosion resistance is possible. However, since the width of printing elements (lines) for practical use is limited to a minimum of the order of 200 .mu.m, it has been difficult to accomplish printing of complicated fine patterns by this method.
The offset printing process typically comprises forming lipophilic parts and hydrophilic parts on a presensitized (PS) plate, causing moisture to be retained in the hydrophilic parts thereby to cause repelling of the lipophilic ink, and printing the resulting ink pattern on a print object. In the practice of the offset printing process, particularly in order to improve the printability, the ink pattern on the PS plate is first transferred onto a rubber blanket and thereafter retransferred onto the print object such as paper.
By this printing method, relatively fine printed elements can be readily obtained. However, for reasons such as the inking method and the two transferring steps, the printed ink film thickness tends to become thin, of the order of 1 to 2 .mu.m. This gives rise to the problem of the occurrence of pinholes and broken printing elements. Furthermore, various attempts are being made to increase the coating film thickness of the ink and to devise processes capable of forming fine patterns of excellent erosion resistance in the practice of this printing method. However, if the ink film thickness is made thick, the printed elements will become wide. Consequently, printing by this method has been limited to a minimum printed element width of the order of 100 to 200 .mu.m.
This is the limit by the above described printing method when fine-line printing is attempted thereby. If printing of a pattern of even finer line widths is attempted, the ink film thickness will simultaneously become thinner. For this reason, it has not been possible to apply this printing method to the forming of particularly fine patterns requiring erosion resistance such as resist patterns.
Thus, by a printing method, it has been difficult to form fine patterns. Moreover, the printed pattern could not necessarily be made true to the printing plate pattern, whereby printing has been inadequate on the point of reproducibility. Accordingly, particularly for forming of fine patterns, it has been unavoidably necessary, in general, to resort to photolithography. Formation of very fine patterns is possible by photolithography. In comparison with printing processes, however, photolithographic processes have been accompanied by the problems of complicated process steps, low productivity, and high cost.
As a mode of printing by which patterns of relatively narrow printing elements (lines) can be printed to produce printed films of large thickness, there is the intaglio printing method. This printing method comprises forming recessed parts conforming to printing elements by engraving or etching on a printing plate such as a copper plate, rubbing somewhat hard ink into these recessed parts, wiping off the ink on the non-printing areas, thereafter laying a print paper on the copper plate, and pressing the paper with considerable force against the plate. The reason for thus applying considerable force on the paper is as follows. Since the ink which has been rubbed into the recessed parts is in a state wherein it is somewhat recessed from the surface of the printing plate, a great pressing force is applied on a soft print object such as paper thereby to force the ink surface into positive contact with and adhesion to the surface of the print object. The ink is thereby transferred accurately and positively onto the print object.
The conventional intaglio printing method is more suitable for printing fine printing elements than the aforedescribed screen printing method or offset printing method. In spite of this, however, the intaglio printing method has a great problem in that printing by this method on the surfaces of hard base materials such as rigid plastics, glass, ceramics, and metal is almost impossible.